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Enemy column. Thunderstorms in the west Pacific drive air from the lower atmosphere into the stratosphere.

Enemy column. Thunderstorms in the west Pacific drive air from the lower atmosphere into the stratosphere.


Does the Sky Have a Faulty Filter?

Just when scientists thought the ozone layer’s worst days were behind it, it turns out they may have been missing a big threat to its health. Soon-to-be-published findings suggest that a natural mechanism that filters air rising to the top of the sky may not work as well as previously thought. If subsequent studies confirm the findings, the faulty filter could also have big implications for global climate.

The sky is divided into two major layers: the troposphere close to Earth’s surface and the stratosphere, which in the tropics begins 17,000 meters above it. In addition to providing the air we breathe and the weather we experience, the troposphere is laced with a compound called the hydroxyl radical—abbreviated OH—which bonds to pollutants like bromines and droplets of sulfates called aerosols. It neutralizes most of them before they can reach the stratosphere, preventing them from damaging the ozone layer and wreaking havoc on global climate.

But new work led by scientists at the Alfred Wegener Institute (AWI) in Potsdam, Germany, suggests that the OH filter may not be as reliable as previously thought. In 2009, AWI scientists conducted a research cruise in the west Pacific, where strong thunderstorms push air from the troposphere up to the stratosphere, serving as a main source of air for the upper layer. Because ozone in the troposphere is a precursor to OH, they deployed weather balloons equipped with measuring devices known as sondes to measure the amount of ozone in the air from the surface to the stratosphere. The more ozone they found, they thought, the higher the level of OH, a chemical notoriously difficult to measure directly.

Normally, the amount of ozone in the stratosphere ranges from 30 to 100 ozone particles per billion air molecules. But the researchers, led by AWI atmospheric scientist Markus Rex, found levels in the west Pacific below 10 ozone particles per billion—so low their instruments couldn’t even get a precise count. “The first sonde I really thought was malfunctioning,” Rex says. “We didn’t expect to see such a deep and wide hole in terms of ozone.” But measurements over a 3000-km-wide swath, and up to an altitude of 15 km, showed that the dearth of ozone reached all the way up to the lower stratosphere. This indicates that OH levels in the troposphere may be much lower than previously thought, and their filtering effect less pronounced, the team will report in an upcoming issue of Atmospheric Chemistry and Physics.

The finding could have broad impacts on our understanding of how the stratosphere works, says James Anderson, an atmospheric chemist at Harvard University. If the troposphere’s OH filter is indeed less effective than scientists thought, he says, the west Pacific would provide "a potentially very important avenue for the injection" of pollutants that could damage the ozone layer. And it could explain why past studies measured higher than expected levels of ozone-damaging chemicals in the stratosphere, Rex says. For example, “we always knew there is more bromine in the stratosphere than we could account for.” A miscalculation of the OH filter’s strength may explain the discrepancy.

The climate implications could be broad as well. Sulfur aerosol pollution, created via coal burning, is skyrocketing in Southeast Asia. In the stratosphere, sulfates provide a temporary cooling mask that spreads globally and lasts a few years. If Rex is right, it could mean that cooling pollutants have an easier route to the stratosphere than previously thought, though his study doesn’t calculate the specific climate impacts of the faulty filter.

Other researchers are unconvinced. Atmospheric scientist Laura Pan of the National Center for Atmospheric Research in Boulder, Colorado, says the ozone measurements are just a suggestion of OH levels in the troposphere, not proof. More data, including measurements of other gases that affect OH levels, can confirm the OH levels, she says. She recently completed a field campaign in the same west Pacific region using aircraft that will give more data on the issue, but results are not yet available. The team from AWI, meanwhile, will soon begin a €5 million project off Palau in the west Pacific, funded by the European Union, to take more measurements of atmospheric chemicals that will help it estimate OH levels better.

*Correction, 28 April, 12:43 p.m.: This item incorrectly asserted that Laura Pan stated that measuring OH directly was required to confirm low levels of the chemical. The item has been updated.